1. Field of the Invention
The present invention relates to a perforation method of forming a through-hole in a wall of a hollow member by laser irradiation, and an apparatus for the perforation.
2. Description of the Related Art
An end portion of a fuel injection nozzle 1 to be installed in automobiles is shown in FIG. 3. The end portion of the fuel injection nozzle 1 has a hollow structure as shown in FIG. 3.
A plurality of injection holes 2 extend though the wall of the end portion of the fuel injection nozzle 1. In recent years, such holes have been formed by using a laser light L.
The end portion is irradiated with the laser light L from the outside. Therefore, the wall of the end portion is melted by the laser light L from the outside to the inside. Finally the inner wall surface is melted to form the injection hole 2.
When the irradiation with the laser light L is continued even for a short time after the inner wall is melted to form the injection hole 2, the laser light L reaches the opposite inner wall at a position on the line extending from the injection hole 2. In this case, the opposite inner wall (opposite wall) is melted at the position as a matter of course. Thus, it is difficult to form a through-hole without damaging the opposite wall in the perforation process using the laser light L.
A method for forming the injection hole in the end portion of the fuel injection nozzle by the laser light by inserting a mirror reflector into the end portion is proposed in Japanese Laid-Open Patent Publication No. 09-066381. In this method, the laser light introduced through the injection hole is reflected by the mirror reflector to a light absorber, whereby the laser light is prevented from reaching the opposite wall.
Further, a method, in which a fluid (particularly a liquid) to the end portion of the fuel injection nozzle is introduced for causing cavitation to scatter the laser light, is proposed in Japanese Laid-Open Patent Publication No. 2001-526961 (PCT).
In the related art described in Japanese Laid-Open Patent Publication No. 09-066381, when the incidence angle of the laser light or the reflection angle of the mirror reflector is not strictly controlled, the laser light is reflected to the inner wall. Thus, in the related art, disadvantageously it is difficult to reliably prevent the reflection of the laser light to the inner wall.
In the related art described in Japanese Laid-Open Patent Publication No. 2001-526961 (PCT), a part of the scattered laser light may reach the inner wall, so that the inner wall may be damaged depending on the output of the laser light.